Throughout this document the term station, such as used in base station, macro station, macro base station, femto base station, and femto station, is intended to denote the communications equipment. The term cell, such as used in femtocell or macrocell, is intended to denote the coverage footprint of a corresponding station or the coverage footprint of a sector of a multi-sector base station.
Broadband wireless cells tend to be UL interference limited. There are many scenarios that can cause UL interference. The most common has historically been cell edge interference in a frequency reuse 1 network (where the same frequencies are used throughout a geographic area) or when neighboring cells share a frequency channel causing co-channel interference.
However, the Long Term Evolution (LTE) wireless standard adds an additional scenario. When a femto base station is present in the macrocell coverage footprint of a macro base station in a frequency reuse 1 network, this can create what is termed in LTE as the near-far problem. If a user equipment (UE) is in the femtocell coverage area of the femto base station, but is in communication with the more distant macro base station it may be transmitting using a very high power, causing excessive uplink interference at the femto base station. Many other uplink interference scenarios exist.
Aside from brute force methods such as using very robust and inefficient modulation and coding schemes, current methods to combat this include coordinated multipoint (CoMP) which contains multiple methods. First, fractional frequency reuse may be used, coordinating the UL resources so that a UE communication with a macrocell and a UE communication with a femto base station that has an overlapping coverage footprint do not transmit on the same subcarriers simultaneously. This technique has been previously used in WiMAX. Second, the two base stations may use beamforming to coordinate the UL resources spatially. Both of these methods require coordinated scheduling. Additionally, using joint reception, both base stations receive the same data from an individual UE using the same subcarriers at the same time. This may additionally be beamformed. The joint reception, of course, uses double the resources since the resources of both base stations are tied up with the same reception.
Fractional frequency reuse is inefficient due to the need for one base station to not schedule UL resources while the other is using them. Beamforming requires significant antenna resources which may not be available on a femto base station. Additionally, there may be times when coordinated scheduling of beamforming cannot be achieved due to the bandwidth and quality of service (QoS)/quality of experience (QoE) needs of interfering UEs, requiring a fallback to fractional frequency reuse.
Other attempts to combat uplink interference include use of Inter-Cell Interference Cancellation (ICIC) techniques. Such ICIC techniques include signaling between base stations to inform other stations about future planned transmissions of a base station, and reporting the interference levels experienced by a base station. These techniques use High Interference Indicator (HII) and an Overload Indicator (OI) respectively. HII technique is not spectrally efficient since it may result in base stations avoiding use of bandwidth resources if they heed the planned transmission information provided by the base station providing the HII. OI technique has the additional drawback that it only reports the exposure to interference level after the exposure has occurred.
There exists a need for a spectrally efficient (e.g., efficient modulation and coding, transmitting using substantially more available time and frequency resources) method to resolve UL interference as an alternative to existing spectrally inefficient solutions which merely mitigate UL interference.